BIOMES OF THE EARTH - GRASSLANDS Part 6 doc

Lions will eat small birds, lizards, and animals as small as mice, but their diet consists mainly of gazelle, antelope, and The spotted hyena Crocuta crocuta inhabits grasslands over mos

times more, and hunts mainly at night Antelope and zebras

are among its prey, but hyenas also feed on carrion There are

three species of hyenas The striped (Hyaena hyaena) and

brown (H brunnea) hyenas are smaller than the spotted

hyena and less social

Hunting dogs and hyenas are impressive hunters, but the

lion (Panthera leo) is by far the most famous meat eater of the

savanna Nowadays lions are found only in Africa and in a

very small part of northwestern India, but at one time they

lived throughout most of Europe and the Middle East

Lions live in family groups, called prides, which comprise

up to three adult males and up to 15 adult females together

with their young Known since ancient times as the “king of

beasts,” a male lion is about four feet (1.2 m) tall at the

shoul-der and 10 feet (3 m) long, not counting the tail, and it

weighs 330–530 pounds (150–240 kg) It is a truly formidable

animal, but in fact the male seldom takes part in the hunt

His job is to defend the family’s territory and keep rival males

away from the females Hunting is left mainly to the

lioness-es Lions will eat small birds, lizards, and animals as small as

mice, but their diet consists mainly of gazelle, antelope, and

The spotted hyena

(Crocuta crocuta)

inhabits grasslands over most of Africa south of the Sahara A highly social animal, it lives mainly by scavenging, but it is also a

formidable hunter.

(Courtesy of Fogstock)

zebra A single lion can kill any of these, but when severallions work together, they can kill bigger prey, such as buffaloand giraffes Lions stalk their prey, slowly advancing untilthey are within about 100 feet (30 m) of the target beforecharging If a lion is lucky—and three of every four lionattacks fail—it will be able to grab its victim or knock it to theground with a blow from its paw before the prey animal hastime to escape When several lionesses hunt together, theytry to surround the prey, cutting off its escape routes.

Cheetahs (Acinonyx jubatus) also stalk their prey, and this

part of the hunt can last several hours Once it charges, ever, a cheetah can outrun its prey It can accelerate rapidly

how-to about 60 MPH (96 km/h), but it cannot maintain thisspeed long Most chases last no more than about 20 secondsand cover about 560 feet (170 m) Cheetahs hunt hares, smallantelope, gazelles, wildebeest calves, and birds, includingostriches Centuries ago wealthy people in the Middle Eastand India kept cheetahs for hunting antelope They are ele-gant animals and a cheetah is easily distinguished from other

Lions (Panthera leo)

resting on the African

savanna On the open

grasslands where there

is little cover, lions

must hunt by stealth.

(Courtesy of Fogstock)

(opposite page) The cheetah (Acinonyx jubatus) is a cat built

for speed It stalks its prey until it is close enough to give chase, when it can reach 60 MPH (96 km/h) over a short distance.

(Courtesy of Fogstock)

cats by its long legs, light and agile build, and small head.

The cheetah’s spine is much more flexible than the spine of

other cats Combined with its long legs, this gives it a very

long stride, and unlike other cats it has claws that do not

retract, which give it a better grip when accelerating

Cheetahs hunt mainly by day, when other cats are resting

in the shade The caracal (Felis caracal), or African lynx, hunts

in twilight and at night It measures two to three feet (60–90

cm) from its nose to the root of its tail and is recognizable by

its long, tufted ears Caracals feed on rodents and small deer

and will also kill domestic sheep, goats, and poultry They are

found throughout the African savanna and much of the

Middle East as far as northwestern India

The jaguarundi (F yagouaroundi) of North and South

America is slightly smaller than a caracal, with a red or gray

coat lacking any patterned markings It is found on savanna

grasslands and in scrub from Arizona to northern Argentina

It feeds on rodents, poultry, frogs, and fish Farther south, the

pampas cat (F colocolo) lives among the pampas grass A

small, stocky cat with a thick, bushy tail, it hunts at night,feeding on small mammals

Grassland birds

Most birds fly, but not all do Flying consumes large amounts

of energy, and several groups of birds have abandoned flightand spend their entire life on the ground This presents themwith a problem but also gives them an advantage Flyingbirds can avoid capture by dogs, hyenas, and cats by escapinginto the air, but flightless birds must find an alternativemeans of defense Consequently, some of those living on the

Hunter and prey: The evolutionary arms race

In order to catch their prey, hunting animals—called predators—must either chase them,

ambush them, or set traps for them Hunting dogs, coyotes, and cheetahs chase theirprey Ambush calls for concealment, and it is the strategy many snakes use Some havemarkings that make them almost invisible against the background Where the ground issoft, certain snakes bury themselves with only their eyes and nose projecting above thesurface When a victim is within range they launch a very fast attack Spiders set traps—their webs Engineers have calculated that if strands of spider silk were the thickness of apencil, a spider’s web could catch and hold an airliner

If they are to evade capture, prey must be wary to avoid ambushes, alert to possibletraps, and able to outrun any hunter that gives chase They can also confuse the enemy.One way to do so is to gather in herds; there is safety in numbers This is partly because ahunter can attack only one individual, and it is almost impossible to select a target from aherd of animals that are not only crowded together and moving together, but swervingerratically from side to side What is more, it is very difficult to approach a herd withoutbeing noticed It may appear that all the animals are feeding, but at any moment there arealways a few with their head raised, alert to any movement If a prey animal detects dan-ger, it starts to run, and so do all the others As they run, many gazelles leap into the air.This leaping alters the outline of the herd and adds to the confusion of the pursuer

open grasslands are fast runners Their advantage is that

since they do not leave the ground there is no restriction on

their weight, and flightless birds can be large and heavy

enough to defend themselves against most attackers

The ostrich (Struthio camelus) of the African savanna is the

fastest, and it is also the world’s largest living bird Newly

hatched, an ostrich chick is about 12 inches (30 cm) tall and

already can run An adult bird is six to nine feet (1.8–2.7 m)

tall, and it can run at 44 MPH (70 km/h) Ostriches have keen

eyesight and never sleep more than 15 minutes at a time

They have to bend their heads to the ground while feeding,

but they look up frequently They are so alert to danger that

grazing mammals tend to keep close to them and use them as

an early warning system There is no truth in the old joke

A herd of animals can also turn and fight When hyenas chase eland, for example, the

eland cows that have calves move ahead of the herd Then the herd turns, and the

ani-mals that have no calves advance on the hunters An eland is a big animal with big, sharp

horns and hard hooves In an encounter with a hyena, the eland usually wins

One of the smartest strategies is to exploit the hunter’s habits Small birds often nest

close to the nest of a bird of prey They get away with this because a bird of prey travels

away from its nest before commencing its hunt, so the small birds close to the nest are

quite safe They are also safe from other predators, because those hunters prefer not to

approach the nest of a bird of prey

Prey animals have other ways to prevent themselves from being eaten They may

make themselves objectionable For example, monarch butterflies are poisonous,

ladybugs are not good to eat, and wasps and bees have stings Poisonous or

in-edible species usually have distinctive markings so predators can recognize them

Edible species can take advantage of this form of advertisement by acquiring similar

markings

Over many generations predators acquire more effective weapons and techniques,

and prey animals acquire better defenses As hunters and the hunted try to keep ahead of

each other, this competition turns into an evolutionary arms race that ends only when it

reaches a stable situation in which the predators are able to catch enough food to survive,

but not so much as to wipe out the prey

about ostriches’ burying their heads in the sand The ideaprobably arose because when an ostrich is crouched on itsnest it holds its head close to the ground, where the head isoften partly hidden Ostriches wander the savanna in smallgroups, feeding mainly on plant material, although theysometimes eat small reptiles.

The emu (Dromaius novaehollandiae) is the Australian

equivalent of the ostrich It is about 6.5 feet (2 m) tall andcan run at about 30 MPH (48 km/h) Emus also swim well.These birds travel in small groups and feed on plant materialand insects They cause considerable damage to farm crops,and many were killed between 1932 and 1965 during a cam-paign to exterminate them They are now protected in mostareas and are found throughout Australia

The ostrich (Struthio

camelus) is the largest

living bird, inhabiting

the savanna grasslands

of eastern and southern

Africa and the area

along the Atlantic

coast of North Africa.

(Courtesy of

Louis Azevedo)

The rhea (Rhea americana) lives on the pampa It is smaller

than the ostrich or emu, standing about five feet (1.5 m) tall,

and can run at up to 37 MPH (60 km/h) Old males usually

live alone, but most rheas live in groups of up to 30 birds

They feed on plant material and insects Male rheas raise the

chicks and will defend them ferociously A rhea has a

power-ful kick and is armed with hornlike spurs on its ankles A rhea

will charge a horse, and gauchos take dogs with them for

pro-tection Rheas have even been known to attack taxiing

air-planes!

Ostriches, emus, and rheas are too heavy to fly, but they are

not the only large birds of the grasslands Cranes, which do

fly, live on the grasslands of every continent except South

America They breed in wetlands, however The draining of

these areas, combined with hunting, have contributed to a

drastic decline in their numbers so that most cranes are now

endangered, although they may be recovering as a result of

sustained conservation efforts The whooping crane (Grus

americana) of North America is one of the largest, standing

about 4.5 feet (1.4 m) tall The demoiselle crane (Anthropoides

virgo), one of the most beautiful of all birds, spends the winter

in Africa and southern Asia and returns to the Eurasian steppe

to breed The crowned crane (Balearica pavonina), a bird about

3.2 feet (1 m) tall, is found over most of the African savanna

No prairie bird has lost the ability to fly, but sage grouse

and prairie chickens are very reluctant to do so and postpone

escape until the very last moment The sage grouse

(Centrocercus urophasianus) lives on the drier short-grass

prairies in the west, and the greater (Tympanuchus cupido) and

lesser (T pallidicinctus) prairie chickens live on the tallgrass

prairie in the east

During the breeding season sage grouse and prairie

chick-ens gather in large numbers in traditional lekking areas Each

male occupies and defends a patch of ground called a lek,

where he spreads his feathers, struts around, and utters loud

calls Females move among the leks, choosing the best

per-former, and the winning male mates with most of the

females

Other birds defend themselves by confusing attackers,

using a version of the herd strategy of grazing mammals

Although they can fly, small birds need to drink, and whilethey are on the ground beside a water hole they are vulnera-

ble Consequently, some birds, such as budgerigars tacus undulatus) and cockatoos (family Cacatuidae) of

(Melopsit-Australia, arrive at water holes in large flocks that whirlaround with individuals flying in all directions, making itvery difficult for a predator to select a target, while belowthem individual birds take turns to drink

Weavers are small birds that knot and weave grass blades tomake intricate nests that hang from the trees of the Africansavanna There are 94 species of true weavers, of which the

red-billed quelea (Quelea quelea) is the most abundant.

Queleas defend themselves with a version of the herd

strate-gy, moving around in flocks numbering thousands At ing time queleas gather in even larger flocks, sometimesnumbering millions, and a single tree can carry several hun-dred nests There are so many birds and they are so vigilantthat predators have little hope of approaching without rais-ing the alarm and sending the flock scattering When thequeleas are airborne a predator becomes confused and aban-dons the hunt Consequently, queleas breed more successful-

breed-ly than most small birds They feed onbreed-ly on seeds—includingthose of wheat, millet, and other farm crops—and damagecrops on a scale that is comparable to the devastation caused

by locusts

Many of the grassland birds are predators that feed on

small animals The secretary bird (Sagittarius serpentarius)—

the name refers to its crest of feathers, which resembles abunch of quill pens—spends most of its time on the ground

It eats a variety of animals, including snakes, which it kills bystamping on them It is a large bird, up to 3.3 feet (1 m) talland with a wingspan of 6.5 feet (2 m), that flies well andnests in the top of acacia trees Secretary birds are found only

in Africa and no other bird is quite like them

Most hornbills eat fruit or insects, but some eat smallmammals and also snakes These include the Abyssinian and

southern ground hornbills (Bucorvus abyssinicus and B cafer, respectively) and the yellow-billed hornbill (Tockus flavi- rostris) These very social birds, named for their large bill, are

found throughout the African savanna

Several eagles also eat snakes The short-toed eagle

(Circae-tus gallicus) is a snake-eating eagle of the steppe, and the

bataleur (Terathopius ecaudatus) is a snake eater of the African

savanna, although this species feeds mainly on carrion The

tawny eagle (Aquila rapax) will also eat carrion and spends

much of its time on the ground It is very widely distributed,

occurring throughout the African savanna and the Eurasian

steppes

Vultures are the most famous eaters of carrion They circle

high in the air, constantly looking for food, and when

hunters take down a prey animal the first vulture to see the

event begins to circle lower Other vultures notice this and

converge on the meal—most vultures locate food by

watch-ing other vultures The vultures land and wait until the killer

has eaten its fill then take the remains Different species eat

different parts of the animal, allowing several species to feed

together without competing, although there is much

squab-bling among individuals Vultures can strip a small animal

such as an antelope to the bone in 20 minutes The two most

common African vultures are Rüppell’s griffon (Gyps ruppelli),

which lives on the African savanna mainly north of the

equa-tor, and the lappet-faced vulture (Torgos tracheliotus), found

south of the equator Although they are highly efficient at

locating food, vultures cannot fly at night, and they are

easi-ly driven away by large mammals such as hyenas and jackals

American vultures are not closely related to Old World

tures, but they live in much the same way The turkey

vul-ture, also called the turkey buzzard (Cathartes aura), is found

throughout the prairies and the South American pampa, and

it is unusual among birds in having a keen sense of smell

This allows it to find carcasses lying on the forest floor, and

American vultures are consequently able to enter forests,

unlike Old World vultures, which have no sense of smell and

live only in open country In South America, where their

ranges overlap, turkey vultures are often seen in the

compa-ny of black vultures (Coragyps atratus) Whenever the two

quarrel, the black vulture wins King vultures (Sarcoramphus

papa), of Central and South America, have a poor sense of

smell and rely on their eyesight, but they find food in the

for-est by following other vultures

Coping with drought

Grasslands have a dry climate, often with a season whenalmost no rain falls, and droughts are common (see “Dry sea-sons and rainy seasons” on pages 51–55) Plants must survivethese periods—and, of course, they do

Water enters a plant through its roots and travels from the

roots to every other part along channels called vessels.

Photosynthesis (see the sidebar “Photosynthesis” on page 85)

is the process by which green plants make sugars from waterand carbon dioxide Carbon dioxide must enter the photo-synthesizing cells, and oxygen, a by-product of photosynthe-sis, must leave those cells Gases are exchanged through tiny

pores, called stomata, in the surface of leaves Stomata can be

open or closed, but while they are open for gas exchange,water can evaporate through them and be lost from the

plant This process is called transpiration Ordinarily the water

is immediately replaced by water drawn up from the soil, but

if the soil is dry, the plant may lose water by transpirationfaster than it can be replenished, with serious consequences.Water fills the spaces inside and between plant cells, makingplant tissues rigid Woody plants have solid stems that keepthem upright even after the plant has died, but grasses andforbs are not woody and without water they wilt—becomelimp When it rains the plants recover quickly, but if theyremain without water for more than a certain length of timethe wilting becomes permanent and the plants die

Plants cannot control the weather or the amount of ture in the soil, but they can reduce the rate of transpiration.Water evaporates fastest when it is exposed to direct sunlight.Consequently, grassland plants tend to have many morestomata on the shaded underside of their leaves than on theexposed upper side This arrangement does not work so wellfor grasses, however, because their long, narrow leaf bladespoint upward and are lit from both sides Instead, on warm,

mois-bright days many grasses, especially the feather grasses (Stipa

species) that are so common on grasslands, roll their leavesinto long tubes, with the stomata on the inside

Plants keep their stomata closed on hot, sunny days Thisprevents water loss by transpiration, but it creates anotherproblem: Food production is disrupted The light-independent

stage of photosynthesis can continue with the stomata

closed, using carbon dioxide that was absorbed earlier when

the stomata were open, but the store of carbon dioxide in the

cells is soon depleted and then another chemical reaction

becomes dominant

Rubisco (ribulose biphosphate carboxylase), the enzyme

that attaches to carbon dioxide at the start of the

light-inde-pendent stage of photosynthesis, will accept either carbon

dioxide or oxygen, and when the concentration of carbon

dioxide falls below a certain level, rubisco takes up oxygen

rather than carbon dioxide It then attaches oxygen instead

of carbon dioxide to RuBP (ribulose biphosphate) The

result-ing process is called photorespiration: photo- because it takes

place in light and respiration because it uses oxygen Unlike

ordinary respiration, however, it releases no energy for the

plant, and it reduces photosynthesis by removing carbon

compounds from the cycle Plants in temperate regions of

the world can tolerate photorespiration because although it

slows the rate of photosynthesis, the hot, sunny conditions

that cause it seldom continue long enough to do serious

harm

That is not the case in the Tropics, and several thousand

species of plants, including corn (maize), sugarcane, and

many grasses of the tropical savannas, have evolved a

modi-fied version of photosynthesis that precludes the problem of

photorespiration When an atom of carbon is added to a

mol-ecule of RuBP, the resulting compound (3-phosphoglycerate)

has three carbon atoms in each molecule; plants using this

version of photosynthesis are known as C3plants Plants that

use the modified version of photosynthesis absorb carbon

dioxide into mesophyll cells lying just below the leaf cells that

contain chlorophyll In the mesophyll cells the enzyme PEP

carboxylase catalyzes a reaction that attaches carbon dioxide

to phosphoenolpyruvate (PEP), producing oxaloacetate, a

compound with four carbon atoms in its molecule

Oxaloacetate is then converted to malate, another

four-car-bon compound Plants using this reaction are known as C 4

plants Malate leaves the mesophyll cells and passes through

passageways between them, called plasmodesmata, to enter

bundle-sheath cells packed tightly around leaf veins Inside

the bundle-sheath cells, the malate gives up its carbon ide, which combines with rubisco and enters the ordinarylight-independent stage PEP carboxylase has no affinity foroxygen, so it can capture carbon dioxide even when the con-centration is very low, and because carbon dioxide accumu-lates in the bundle-sheath cells, its concentration there isalways high enough to ensure that it wins the competitionwith oxygen for rubisco, thus preventing photorespiration A

diox-C4plant is able to perform photosynthesis efficiently in hot,sunny weather when its stomata are closed—conditions inwhich C3plants suffer stress

At the start of the rainy season the grassland is ablaze withcolor, which reveals an alternative strategy by which plantssurvive drought Annual plants produce seeds that lie dor-mant in the soil throughout the dry season but sprout veryquickly once the soil around them is moist The plants grow,flower, and produce a new crop of seeds during the rainy sea-son, then die In fact, these plants do not survive drought—they avoid it

Many grassland animals also avoid drought They do so bymigrating in search of water and the better pasture thatgrows where the ground is moister (see “Mammal migra-tions” on pages 155–157)

Many of the African grazers can survive long periods out drinking, obtaining all the liquid their body needs from

with-the vegetation with-they eat The sassaby or tsessebi (Damaliscus lunatus), for example, can live without drinking for 30 days Thomson’s gazelle (Gazella thomsonii) drinks only when the pasture is very dry The haartebeest (Alcelaphus buselaphus)

can also survive for a long time without water, although itdrinks readily when water is available

Some animals do not need to drink at all These include

the Beira antelope (Dorcatragus megalotis), springbok dorcas marsupialis), gerenuk (Litocranius walleri), and Grant’s gazelle (Gazella granti) Their ability to retain water, thereby

(Anti-making do with very little, allows animals such as these toventure into the driest parts of the savanna, where most ani-mals would perish from thirst, and they have no need tomigrate in search of water during the dry season This confers

an added advantage: The predators that hunt them during

the rainy season do need to drink, and they follow the

migrating herds in search of water, leaving the nondrinkers

in peace

Coping with heat and cold

Animal bodies function only within a specific range of

temper-atures The normal body temperature of a person is between

96.4°F (35.8°C) and 99.5°F (37.5°C) If the temperature rises or

falls outside this range the body will respond in ways aimed at

moving the temperature back within the tolerable range, and

if the temperature wanders far outside this range the person

will become severely ill Death is likely if the body temperature

falls below 78.8°F (26.0°C) or rises above 109.4°F (43.0°C)

Most mammals have a similar tolerable range Birds have a

higher average temperature, of about 104°F (40°C)

Birds and mammals are able to maintain a constant body

temperature by internal means If we are cold, we shiver, for

example, thus warming the body, and if we are hot, we sweat,

cooling the skin by allowing the sweat to evaporate Reptiles

are unable to shiver, sweat, or control their body temperature

in any other internal way Instead, they must absorb warmth

from outside the body when they are cold and find cool

sur-roundings when they are hot

Birds and mammals are sometimes described as

“warm-blooded” and reptiles as “cold-blooded,” but this description

is misleading, because while a reptile’s body is active it is

quite warm—sometimes warmer than the body of a

mam-mal Reptiles are active when their body temperature is

between about 88°F (31°C) and 100°F (38°C) They are unable

to move if their temperature falls below about 45°F (7°C) or

rises above 109°F (43°C) Birds and mammals are more

accu-rately described as endotherms and reptiles as ectotherms;

endo-means “internal” and ecto- endo-means “external.” Amphibians

and fish are poikilotherms.

On the tropical savanna most animals seek shade during

the hottest part of the day They remain fairly still, resting

Reptiles and small mammals retreat into burrows Toward

evening, as the temperature falls, they emerge to feed, and

soon after dusk, as the temperature falls still further, many

become inactive once more Early in the morning most mals enjoy feeling the warmth of the rising Sun Reptilesneed that warmth and must bask in the sunshine until theirmuscles reach working temperature.

ani-Basking and sheltering make it possible to control bodytemperature within fine limits If a lizard is too warm, it willmove alternately from sunshine to shade or turn to face intothe Sun to minimize the area of its body that is directlyexposed This prevents its body from overheating Wherethere is no shade, many large mammals that rest during themiddle of the day turn from time to time so they are con-stantly facing the sun, rather than letting it beat down ontheir flanks, thus minimizing the area of body surface direct-

ly exposed to the sun They may also choose to lie closetogether, so they shade one another

Some animals adopt a more radical strategy for avoiding

extreme heat They enter a condition called torpor, in which

they lose consciousness, their breathing and heartbeat slow,

and their temperature rises This technique, known as tion, allows the animals to survive prolonged periods of high

estiva-temperature and drought; it is more common among desertanimals than grassland species, however

Animals living on the temperate grasslands have no need

to escape the extreme summer heat of the savanna Instead,they must survive long, cold winters As the temperaturestarts to fall reptiles retreat for the winter into secure hidingplaces, sheltered from the wind, where the temperature isunlikely to fall so low that they die Once the temperaturefalls below 45°F (7°C), reptiles become immobilized andutterly helpless Winter temperatures define the boundaries

of the regions reptiles can inhabit, although the adder (Vipera berus) and common garter snake (Thamnophis sirtalis) live in

latitudes up to about 68°N

Birds often have difficulty maintaining their high bodytemperature when the air temperature is very low Those thatare active by day must find enough food to provide the ener-

gy to keep them warm through the night, and a bird that fails

to do so may not survive Consequently, many birds solvethe problem of winter cold by migrating